To produce various topological structures in a silicon substrate, for example passivation trenches in a power thyristor, use is frequently made of isotropically acting acid mixtures.
The commonest acid mixtures are composed of hydrofluoric acid (HF) and nitric acid (HNO.sub.3) and are diluted with acetic acid (CH.sub.3 COOH or HC.sub.2 H.sub.3 O.sub.2), water or other additives. As is known from the paper by B. Schwartz and H. Robbins, "Chemical Etching of Silicon", J. Electrochem. Soc., Vol. 123, No. 12 (December 1976), pages 1903-1909 (see FIGS. 8 and 9 therein), the composition of the acid mixture determines the etching rate, but also the topological structure of the etched surface or the pattern of the contours produced if etching is carried out with masking.
As is evident from FIG. 8 of the publication cited, the etching rate reaches a maximum if HF and HNO.sub.3 are mixed in a ratio of about 7:3 and are consequently in stoichiometric equilibrium.
On the basis of such a mixing ratio, the etching rate can be controlled by adding CH.sub.3 COOH. If the acid mixture contains more than 40% of acetic acid, the etching rate finally drops virtually to zero.
As already mentioned, the structure of the etched surface is likewise determined by the composition of the acid mixture. In this connection, it is evident from the corresponding FIG. 9 of the cited publication that the abovementioned stoichiometric composition does not yield a desirable surface quality. In addition, the high etching rate associated with this composition is not exactly desirable since the etching process is more difficult to control.
If it is desired to etch in a controlled manner and to achieve smooth and bright surfaces, the HF:HNO.sub.3 ratio has to be altered in favor of nitric acid. The reason for this is that the solution of the resultant SiO.sub.2 produced by the hydrofluoric acid consequently becomes the factor limiting the etching rate, and crystal defects, doping and orientation have almost no effect on the etching rate. The HNO.sub.3 -rich etching mixtures have, in particular, a polishing action.
If it is then desired to etch structures in a silicon substrate, the parts of the surface which are not to be etched have to be coated (masked).
In this connection it is known (see, for example, German Offenlegungsschrift No. 3,334,095) that use is made for such masking of photoresists which are applied, with or without additional adhesion layer, to the silicon substrate and are then photolithographically structured. If photoresist is used, however, a further factor, which limits the choice of the acid mixture, has to be taken into consideration, since the photoresist is in general attacked by HNO.sub.3.
To etch the structures with a fineness of down to 10 .mu.m or still finer, a negative-working photoresist is normally used as etching mask because positive-working photoresists are unusable as etching mask because of their considerably lower resistance to the usual acid mixtures.
All in all, the use of photoresists has a number of disadvantages:
(1) The choice of the acid mixture is considerably limited. The HF:HNO.sub.3 should be greater than 1:3. Acid mixtures which contain 100% strength HNO.sub.3 and yield reflecting surfaces attack the photoresist within a few seconds and the etching process cannot therefore be masked in this manner. PA1 (2) The resistance of the photoresist, even to less agressive acid mixtures, is dependent on several factors (such as, for example, resist thickness, predrying, exposure, resist age) and the etching bath temperature has to be lowered in order to extend said resistance. PA1 (3) Like the resist resistance, the resist adhesion (in particular without additional adhesion layer) is dependent on the photolithographic process, but also on the surface quality of the silicon substrate. As a rule, the resist adhesion is greater on a rough silicon surface. The masking of a polished surface, which is specifically desirable for other reasons, presents considerable problems (on this point see also German Offenlegungsschrift No. 3,334,095). In particular, the edge of the resist is severely stressed since, as a result of the underetching, the acid acquires access to the lower soft layers of resist and consequently brings about an accelerated underetching. PA1 (4) The surface life of the resist starts to expire with the initial immersion in the acid mixture. The surface life of the resist in the acid mixture is only a few minutes and is shortened in a rather undefined manner by interrupting the etching process so that controlled continuation of the etching process is not possible after an interruption. PA1 (5) In the photoresist process, extremely thin resist residues are left behind after the removal of the unexposed resist areas by the developer. Said resist residues produce a nonuniform removal by etching in the initial etching phase, and this results in a rough surface. In addition, this roughness does not disappear completely because the etching mixtures concerned have weaker polishing characteristics. PA1 (6) During the etching, the photoresist swells and the acid mixture may be degraded by the organic substances. PA1 (7) After the etching process, the resist has to be removed, and this is again carried out with organic substances. These contaminate the freshly etched silicon surface, and this has a very adverse effect on the electrical characteristics of the semiconductor structures produced and makes a complicated posttreatment necessary. PA1 (1) The choice of the acid mixture is unlimited in the HNO.sub.3 direction since the SiO.sub.2 mask layer is attacked only by HF. Even HNO.sub.3 -rich polishing and otherwise unmaskable acid mixtures (including those containing 100% strength HNO.sub.3) can therefore be used. Even positive-working photoresists can be used for structuring the SiO.sub.2 layer. The etching of the SiO.sub.2 layer itself is in any case a largely established and satisfactorily mastered technology. PA1 (2) The resistance of the SiO.sub.2 layer is determined only by the acid mixture. The Si:SiO.sub.2 etching ratio is even temperature-dependent so that the etching can be carried out within a large temperature range. PA1 (3) The adhesion of the mask layer does not present any problems, in particular if the SiO.sub.2 is thermally grown in accordance with a preferred embodiment of the invention. Thermally grown SiO.sub.2, in particular, forms a compact material with the silicon substrate. A polished surface is of advantage in this case, because the SiO.sub.2 layer is then particularly uniform. The mask edge is perfectly protected by the etching process itself: the etching mixture, which employs an excess of HNO.sub.3, is depleted locally of the HF constituent by the etching process above the unmasked Si. As a result of this, the removal of the SiO.sub.2 mask layer is reduced at this exposed point. This reduced etching rate at the mask edge has actually been observed in experiments and, depending on etching mixture and structure, is 9-14% below the etching rate on a continuous SiO.sub.2 layer. PA1 (4) The service life of the SiO.sub.2 mask layer is not affected uncontrollably by an interruption in the etching process. The etching process may be interrupted several times for inspection purposes without the mask suffering as a result. This has the advantage, moreover, that it is possible to transfer to another etching bath after the interruption and the choice of the other etching solution can be subordinated to other criteria (for example, leakage current reduction). PA1 (5) The SiO.sub.2 mask layer permits all the usual cleaning procedures for the silicon substrate which are necessary to remove organic and metallic impurities. Organic substances are no longer present in the etching process itself so that the acid mixture cannot be degraded. PA1 (6) In principle, the SiO.sub.2 mask does not have to be removed immediately after the etching process: since the structure involving the SiO.sub.2 layer is very resistant, other processes (for example the passivation) may follow, and the removal of the SiO.sub.2 mask or parts thereof at points at which, for example, contact is to be made, may be postponed to a subsequent point in time in the process sequence. PA1 (7) In etching complicated structures with varying depth profile, in particular, the desired structure can be prefabricated on a reduced scale in terms of thickness in the SiO.sub.2 mask layer so that only etching steps follow, without the need for remasking in between. Masking and etching are consequently separate parts of the process. The masking itself is carried out on the plane surface and is therefore not impaired by structures already present.
In addition to these disadvantages generally associated with a photoresist mask, further problems occur if recesses which have a varying depth profile are to be etched into the silicon substrate.
Such recesses with varying depth profile may, for example, be trenches with a bevel such as are known from German Patent No. 2,359,511 or stepped double trenches (socalled "double moats") such as are known as passivation trenches from EP-A2-0,178,387 (FIG. 2).
The process for producing the bevelled trench known from German Patent No. 2,359,511 relies on a very specific etching mixture and orientation of the silicon substrate and is unsuitable, in particular, for producing recesses with other types of depth profiles (for example, the double trenches).
On the other hand, in etching a double trench, for example, a masking step would have to be included yet again between two etching steps (for each of the trenches), in which case the already structured surface presents problems in applying (spinning on) and developing the photoresist.